1. Field of the Invention
The present invention relates to a sheet feeding device having a first feeding roller and a second feeding roller and configured to feed a sheet member on the basis of a commanded feeding amount included in each of successively generated feeding commands, an image recording apparatus including the sheet feeding device, and a method of compensating a feeding amount of the sheet member.
2. Description of Related Art
A known image recording apparatus is shown in FIGS. 7A-7C. This image recording apparatus has a carriage 201 on which a recording head 200 is mounted, and is provided with a sheet feeding device 210 for feeding a recording sheet S toward a platen 202. Similar known sheet feeding devices are shown in JP-8-90858A, JP-2006-272772A and JP-2007-63016A. The sheet feeding device 210 includes a feed roller pair 203 disposed on an upstream side of the platen 202 as viewed in a feeding direction of the recording sheet S, and a discharge roller pair 204 disposed on a downstream side of the platen 202 as viewed in the feeding direction.
As shown in FIGS. 7A-7C, the feed roller pair 203 consists of a drive roller 205 driven by a torque received from a drive motor, and a driven roller 206 biased by a coil spring 207 for pressing contact with the drive roller 205 and rotated with the drive roller 205. After the leading end of the recording sheet S has reached a pressure nip between the feed roller pair 203, a feeding movement of the recording sheet S is initiated with the leading end portion being nipped by and between the drive and driven rollers 205, 206. During this feeding movement, the recording sheet S is pressed with a biasing force of the coil spring 207.
After the recording sheet S is fed by the feed roller pair 203 by a predetermined distance, the leading end portion of the recording sheet S reaches a pressure nip between the discharge roller pair 204, and the recording sheet S is then fed by both the feed roller pair 203 and the discharge roller pair 204, as shown in FIG. 7B. Like the feed roller pair 203, the discharge roller pair 204 consists of a drive roller 208, and a driven roller 209 biased by a coil spring 211 for pressing contact with the drive roller 209. The discharge roller pair 204 contacts the recording sheet S after an image is recorded on the recording sheet S, so that the image may be deteriorated if the pressure of the pressure nip between the discharge roller pair 204 is excessively high. To prevent this deterioration of the image, a biasing force of the coil spring 211 for the driven roller 209 is selected to be smaller than that of the coil spring 207 for the driven roller 206.
When the recording sheet S is further fed by the feed roller pair 203 and discharge roller pair 204 by a predetermined distance, the trailing edge of the recording sheet S leaves the pressure nip between the feed roller pair 203, and the recording sheet S is fed by only the discharge roller pair 204, as shown in FIG. 7C.
It is desirable or ideal that an amount of feeding of the recording sheet S by the drive roller 205 and an amount of feeding of the recording sheet S by the drive roller 208 are made equal to each other. To this end, a power transmission coefficient of a power transmission mechanism from a drive power source such as a drive motor to the drive rollers 205, 208, and outside diameters of these drive rollers 205, 208 are determined such that the amounts of feeding by the two drive rollers 205, 208 per unit amount of operation (unit angle of rotation of a rotary member) of the drive power source are equal to each other. However, even if nominal peripheral speeds of the two drive rollers 205, 208 are designed to be equal to each other, dimensional errors of the drive rollers 205, 208 and the related components of the power transmission mechanism will cause a difference between the actual peripheral speeds of the drive rollers 205, 208. Namely, the actual peripheral speeds of the drive rollers 205, 208 are not usually equal to each other. In some sheet feeding devices, the peripheral speeds of the two drive rollers 205, 208 are intentionally made different from each other, for the purpose of preventing a flexure of the recording sheet S while the recording sheet S is fed by both of the two drive rollers 205, 208. Where the peripheral speed of the drive roller 208 is higher than that of the drive roller 205 in the known sheet feeding device 210, a tensile force T acts on the recording sheet S in the feeding direction (in the horizontal direction as seen in FIG. 7), while the recording sheet S is fed by both the feed roller pair 203 and the discharge roller pair 204, as indicated in FIG. 7B. This tensile force T has the largest value corresponding to a force of friction between the feed roller and discharge roller pairs 203, 204 and the recording sheet S. The friction force between the roller pairs 203, 204 and the recording sheet S is generated by a force of pressing contact of the roller pairs 203, 204 with the recording sheet S, and is almost proportional to this force of pressing contact. The tensile force T acting on the recording sheet S causes deformation of surface portions and shaft portions of the drive roller 208 and driven roller 209 of the discharge roller pair 204, and deformation of the power transmission mechanism from the drive motor or other drive power source to the drive roller 208. The tensile force T becomes absent or is zeroed when the force of pressing contact of the feed roller pair 203 with the recording sheet S is zeroed immediately after the trailing edge of the recording sheet has left the feed roller pair 203. When the tensile force T is zeroed, the deformed shaft portions of the discharge roller pair 204 and the deformed power transmission mechanism are restored to their original non-deformed states, with displacements of the shaft portions of the discharge roller pair 204 and a rise of the rotating speed of the drive roller 208, causing an excessive feeding movement of the recording sheet S by a distance larger than a distance corresponding to the commanded feeding amount. This excessive feeding movement will be simply hereinafter referred to as “an overrun” of the recording sheet S. As a result, the recording sheet S suffers from a so-called “hunting phenomenon” (or “image forming failure”), which considerably deteriorates the quality of the image formed by the image forming apparatus provided with the sheet feeding device.